In practicing the invention according to my parent application, a hollow mandrel was inserted into a pre-drilled hole. The mandrel had an opening with a chute at one side near its top through which the concrete was poured, and an opening at its top through which re-inforcing steel was inserted. Both the chute and the top opening were closed by power-operated doors so that the interior of the mandrel could be pressurized while a lifting force was applied to it. While that apparatus was successful and a substantial improvement over the prior art, particularly in that it provided for the insertion of re-inforcing steel into the pile being formed, it had a drawback in that cement particles lodged in faces of the door seals and deleteriously effected the pressurization of the interior of the pile and, in addition, permitted blow-out of concrete through the cracks. The object now is to entirely eliminate the doors and the power mechanisms for operating them and to use instead a simple but completely effective device for sealing off the interior of the pile below the chute and, alternatively, for engaging in the top of the mandrel for steadying it while concrete is poured in through the chute.
In accordance with the foregoing objectives, it is intended now to provide an expansible closdure which can be lowered into the upper portion of the mandrel to seal off the mandrel below the chute opening, and a compressed air conduit extending through the closure so as to force air into the mandrel interior. While concrete is poured into the mandrel via the chute, the closure is deflated and withdrawn to above the chute opening and, if desired, the closure can then be expanded in the top of the mandrel and, with the upper end of the closure adjacent a crosstree in the leads, the closure may be used to steady the mandrel. When reinforcing steel is to be inserted, the closure is deflated and lifted away so that the steel can be inserted through the then-open top of the mandrel.